Molecular rotors are known for their viscosity-sensitive fluorescence quantum yield. This group of fluorescent dyes consists of two molecular subunits (an electron donor group and an electron acceptor group) that are linked through a single bond. Intramolecular rotation or twisting around that single bond is possible. This rotation, a thermally-induced effect, leads to non-fluorescent de-excitation. Inhibition of this intramolecular rotation, typically seen in solvents with high viscosity, causes a shift towards radiative relaxation. As a consequence, fluorescence quantum yield increases in media with increasing viscosity.
A novel effect has been observed that molecular rotors with a polar group dissolved in a polar solvent, exhibit a strong increase in quantum yield when exposed to solvent velocity gradients. This effect allows designing of fluorescence-based shear stress sensors in arbitrary flow fields.
The observation of velocity fields is a difficult task. Often, high-speed microscope imaging is used to obtain time-lapse frames of microspheres suspended in the fluid. Their displacement between frames allows the computation of the flow field. Additional methods include Doppler ultrasound, laser-Doppler velocimetry, and imaging methods based on magnetic resonance. All methods are fairly complex and highly susceptible to noise, particularly at low flow rates. In addition, MRI and ultrasound methods suffer from low spatial resolution, and imaging equipment (especially MRI) is prohibitively expensive.
Shear stress, the product of fluid viscosity and shear rate, is impossible to obtain by noninvasive imaging-based methods. Mechanical methods can be devised to obtain information on the drag forces of a viscous fluid, but they disturb the flow field and require a complex mechanical setup involving bodies susceptible to drag attached to the base through elastic linkers. These methods are not established and are limited to laboratory geometries.